This invention relates to fuel cell systems and, in particular, to fuel cell systems which include gasifiers for generating fuel gas for the systems.
In a conventional high temperature fuel cell system, such as, for example, an internally reforming carbonate fuel cell system, efforts have been made to integrate a coal gasifier with the fuel cell system for generating fuel gas for the fuel cell of the system. In a system of this type, the fuel gas generated by the coal gasification process is primarily in the form of hydrogen, carbon monoxide and carbon dioxide. This fuel gas is fed to the anode compartment of the fuel cell, while the cathode compartment receives oxidant gas or air. In the anode compartment of the fuel cell, internal reforming of any hydrocarbons present in the fuel gas occurs. The reformed fuel gas in the anode compartment and the oxidant gas in the cathode compartment, in the presence of the carbonate electrolyte of the fuel cell, then undergo electrochemical conversion to generate electricity.
In a fuel cell system of the above type, waste heat carried by the exhaust gases from the cathode compartment is used to generate steam in a heat recovery steam generator. The resultant steam is then used in a so-called steam turbine bottoming cycle, where additional power is produced, but at a lower efficiency than in the fuel cell.
The steam generated in the heat recovery system is also supplied to the coal gasifier to provide the steam needed for the gasification process. The latter process when conventionally carried out requires a high temperature for complete conversion of the hydrocarbon coal into a tar-free fuel gas. In order to realize this high temperature, oxidant gas is usually fed to the gasifier to burn a portion of the coal and thereby generate heat.
The high temperature used in the gasification process, results in a fuel gas whose temperature is generally higher than that desired for the equipment which is to process the gas. This requires the gas to be cooled, resulting in efficiency and cost penalties. Similar penalties result from the use in the gasifier of oxidant gas to burn coal to generate heat for the gasification process.
It is therefore an object of the present invention to provide a fuel cell system including a gasifier which attempts to overcome the above disadvantages.
It is a further object of the present invention to provide a fuel cell system including a gasifier in which the need to provide oxidant to the gasifier to generate heat is avoided.
It is yet a further object of the present invention to provide a fuel cell system including a gasifier in which the gasification process is carried out in a manner to promote the overall efficiency of the fuel cell.